Transmission-circuits



v. DEMAREST.

TRANSMlSSWN CIRCUITS.

APPLICATION FILED MAY n, ma.

Patented Feb. 8, 1921.

vINVENTOR UNITED STATES PATENT OFFICE.

CHARLES S. DEMAREST, OF BROOKLYN, NEW YORK, ASSIGNOB TO AMERICAN m1:- PHONE AND TELEGRAPH COMPANY. A CORPORATION OF NEW YORK.

TRANSMISSION-CIRCUITS.

Specification of Letters Patent.

Patented Feb. 8, 1921.

Application filed May 17, 1919. Serial No. 297,910.

T 0 all who m it ma 1 concern Be it known that I, CHAnLns S. DiiMAnns'r, residing at Brooklyn, in the county of Kings and State of New York, have invented certain Improvements in Transmission-Circuits, of which the following is a specification.

This invention relates to transmission circuits and more particularly to signaling arrangements associated therewith.

In a transmission system in which two circuits of unequal impedance values are con led together certain transmission losses will occur which are commonly termed reflection losses. In order to provide ideal transmission conditions and to avoid such reflection losses it is desirable to approximately equalize the impedance values of the two circuits. This is, in practice, accomplished by including in the arrangements interconnecting the two circuits an autotransformer so adjusted as to give the desired im edance ratio between the two cir cuits. VFhile the autotransformer might be adjusted to give a desirable impedance ratio between the two circuits when currents of a certain frequency, such as telephonic currents, were being transmitted over the sys tem, still such an adjustment of the autotransformer might result in a very undesirable impedance ratio between the circuits whcn currents of other frequencies, such as signaling currents, were bein transmitted over the system, due to the di erence in impedance values between the equipment, such as the telephone and signaling equipment, associated with the system and also due to the construction and electrical characteristics of the transformer itself. A specific illustration wherein the above conditions would exist would be in a telephone system in which two stations are interconnected by 7 will have a desirable relation to the impedance of the terminal circuit, which for purposes of illustration may be assumed to have an impedance of approximately 800 ohms. Associated with the terminal circuit is a circuit including signaling or ringing equipment. As the impedance of this terminal signaling circuit might be relatively higher than that of the terminal circuit itself, as for example approximately 5000 ohms, it will be seen when signaling current is transmitted over the system that to obtain a desirable impedance ratio between the repeater circuit and the terminal signaling circuit it would not be desirable to step down the impedance of the repeater circuit but rather to step up such impedance, or to remove the transformer which served to step down the impedance in the case of telephonic currents. Accordingly, in former arrangements of this character, it has been customary to associate with the repeater circuit and with the terminal circuit relay means whereby the signaling currents transmitted over the system might be transmitted around the autotransformer, so that the auaccordingly reduce the number of relays associated with such a system. In the arrangements of this invention this is accomplished' by connecting to the autotrans. former a circuit resonant to currents of certain frequencies, such as signaling currents. Accordingly the transformer will be antiresonant to currents of these frequencies and such currents will be transmitted over the system with a result approximately the same as if the transformer had been eliminated from the circuit and the normal impedance ratio between the interconnected circuits will be maintained. However the transformer will be resonant to currents of other frequencies, such as telephonic currents, and when such currents are transmitted over the system the autotransformer will be rendered effective to establish an impedance relation between the circuits other than that normally existing. @ther and further features of the invention will be clear from the detailed description of the invention which follows.

In the drawing is shown a circuit diagram embodying the arrangements of the invention. For purposes of illustration the ar rangements are shown as associated with one of the. terminal. arrangements of the well known four-wire circuit in which two lines L and L suitable for transmission in opposite directions and including one way repeaters (not shown) are connected by means of a three winding transformer 13 to a terminal line L. As the system may be used for telegraph as well as telephone purposestho telegraph conductors 1 and 15 are associated with the lines L and L respectively by means of the composite sets 16 and 17.. Associated with the terminal line L is apparatus responsive to signaling currents. The signaling currents transmitted over lines L and L must be of a high enough frequency so as not to cause interference with the telegraph apparatus associated with these lines. It has been found in practice that signaling currents having a he quency in the neighborhood of 135 cycles are satisfactory for this purpose. Accordingly the signaling apparatus associated with line L must be adapted to translate the incoming signalingcurrents such as 135 cycle currents, into currents of relatively lower frequency as the signaling apparatus associated withother portions of the system, such as switchboard apparatus, is tuned to respond to currents of frequencies in the neighborhood of 20 cycles. Accordingly there is as sociated with the line L a relay 2 which responds to the signaling current transmitted out over the line from the switchboard apparatus. The operation of relay 2 causes :the energization of relay 6 which connects to the line a source 8 of signaling current suitable for transmission out over the line L which current might in practice have a frequency" in the neighborhood of 135 cycles. The signaling currents coming in over the line L will be transmitted through a bridge across the sides of line L and through the vacuum bulb amplifier 5 whereby they will be amplified to a desired magnitude and will thence be transmitted about the winding of relay 4 and will operate said relay. The operation of relay 4: will cause relay 1 to be energized which will connect to the sides of line L a source 7 of signaling current suitable for operating apparatus at the switchboard, which signaling current might in practice have a frequency in the neighborhood of 20 cycles. Included in line L between relays 2 and 4 is the filter apparatus 3 which serves to attenuate and practically extinguish currents of frequencies suitable for signaling purposes but which readily allows the transmission of currents of telephonic frequencies. Accordingly the outgoing signaling currents will not be able to cause interference with relay 4 and the incoming signaling currents will not be able to cause interference with relay 2. Associated with the windings of transformer 13 is the artificial line or network N which serves to balance the line L. y

Associated with the repeater circuit L and the terminal circuit L is the autotransformer 12. Normally a difference exists betweenv the impedance values of the repeater circuit L and the terminal circuit L. For example the impedance of the repeater circuit L might be approximately 1500 ohms and the impedance of the terminal circuit L (disregarding the signaling apparatus associated therewith) might be approximately 800. ohms. Accordingly the autotransformer 12 is associated with these circuits and adjusted to step down the impedance of the repeater circuit, so that a desirable impedl ance ratio may be effected between the two circuits and reflection losses obviated. The autotransformer 12 is adjusted to efiiciently transmit currents of telephonic frequencies and accordingly when such currents are transmitted over the system they will be transmitted through the autotransformer 12 and it will be rendered effective with respect to such currents to produce a desired impedance ratio between the two circuits.

A diflerence also may exist between the impedance values of the repeater circuit L and the terminal circuit L when signaling currents are transmitted over the system and the signaling apparatus is included in the terminal, circuit. For example, the imped-' ance of the repeater circuit L might be approximately 1500 ohms and the impedance of signaling apparatus included in the ter-, minal circuit L might be approximately 5000 ohms. In such a case, when signaling currents were being transmitted over the system, it would not 'be desirable to-interconnect the circuits L and L by means of an autotransformer which would step down the impedance of the circuit L but better transmission would take place if the autotransformer 12-were removed from the system. Accordingly there is associated with the autotransformer 12 a circuit 10 which is resonant to currents of frequencies suitable for signaling purposes. .With this arrangement the autotransformer 12 itself will be anti-resonant to currents of signaling frequencies. Accordingly such signaling currents will be transmitted over the circuits L and L in a manner substantially the same as if the autotransformer 12 had been eliminated from the circuit. By this means the impedance of circuit L will not be stepped down when signaling currents are transmitted over the system and a more desirable impedance ratio will be maintained between circuit L. and circuit L when the signaling apparatus is associated therewith.

Associated with line L is the autotransformer ll which serves with respect to telephonic currents to effect a desirable impedance ratio between circuit L and circuit L Associated with autotransformer 11 is a circuit 9 which is resonant to currents of signaling frequencies. As autotransformer 11 and circuit 9 are substantially similar to autotransformer 12 and circuit 10 no further description thereof will be given.

It is pointed out that while the autotransformcr arrangements of this invention have been specifically illustrated as included in a four-wire telephone circuit, that they are not limited to use with such circuits but may be employed in the arrangements whereby many other types of circuits are interconnected Furthermore while the arrangements have been illustrated as suitable to effect a different impedance ratio between two circuits for currents of telephonic frequencies than for currents of signaling frequencies, it is understood that the resonant circuits, such as 9 and 10, may be adjusted so that the arrangements will operate suitably with currents of other frequencies. Accordingly the arrangement is capable of embodiment in many and other widely varied forms aS defined in the appended claims.

What is claimed is:

17 A transmission system including two circuits interconnected through an autotransformer, said autotransformer being anti-resonant to currents of certain frequencies.

2. A transmission system including two circuits interconnected through an autotransformer, and a circuit resonant to currents of certain frequencies included in said a utotransformer.

3. A transmission system including two circuits normally of different impedance and interconnected through an autotransformer, said autotransformer being resonant to currents of certain frequencies and efl'ective when said currents are transmitted over said circuits to establish between said circuits an impedance ratio other than that normally existing, and means associated with said autotransformer whereby said autOtXaIlsfOI'Inel will be anti-resonant to currents of other frequencies and when said last mentioned currents are transmitted over said circuits will allow the normal impedance ratio existing between said circuits to be maintained.

In testimony whereof, I'have signed 111 name to this specification this 16th day of May, 1919.

CHARLES S. DEMAREST. 

